The present invention relates in general to substrate manufacturing technologies and in particular to apparatus for determining a temperature of a substrate and methods therefor.
In the processing of a substrate, e.g., a semiconductor substrate or a glass panel such as one used in flat panel display manufacturing, plasma is often employed. As part of the processing of a substrate for example, the substrate is divided into a plurality of dies, or rectangular areas, each of which will become an integrated circuit. The substrate is then processed in a series of steps in which materials are selectively removed (etching) and deposited. Control of the transistor gate critical dimension (CD) on the order of a few nanometers is a top priority, as each nanometer deviation from the target gate length may translate directly into the operational speed of these devices.
Areas of the hardened emulsion are then selectively removed, causing components of the underlying layer to become exposed. The substrate is then placed in a plasma processing chamber on a substrate support structure comprising a mono-polar or bi-polar electrode, called a chuck or pedestal. An appropriate set of process gases are then flowed into the chamber and struck to form a plasma to etch exposed areas of the substrate.
However, with these and other plasma processes, it is often difficult to monitor the process since process conditions may be dynamic within a plasma processing system because of chamber residue build up, plasma damage to chamber structures, etc. For example, in order to enhance the uniformity of plasma processing of a substrate in a plasma processing apparatus, it is desirable to control the temperature at exposed surfaces of the substrate at which etching occurs, on which material is deposited (e.g., by a CVD or PVD technique), and/or from which photoresist is removed. If a substrate's temperature rises above a certain temperature, substrate damage (e.g., photoresist damage) can occur, and temperature-dependent chemical reactions can be altered. Substrate temperature may also significantly affect plasma selectivity by changing the deposition rate of polymeric films, such as poly-fluoro-carbon, on the substrate surface. Careful monitoring may minimize variation, allow a wider process window for other parameters, and improve process control. However, in practice it may be difficult to directly determine temperature in-situ without affecting the plasma process. Hence indirect temperature measurement (ITM) devices that do not physically contact the substrate are preferred.
However, some ITM devices may not be able to sufficiently isolate a substrate temperature from thermal energy propagated by other structures (e.g., chuck, etc.) in close proximity. For example, a thermocouple may be attached to a chuck which is, in turn, in thermal contact with a substrate. The chuck, with a difference thermal mass than the substrate, may also be at a different temperature. Hence, the ITM device must generally be calibrated for a particular substrate configuration in a particular process recipe.
In Contrast, other ITM devices may be able to thermally isolate the substrate, but are themselves sensitive to physical variations between substrates that will affect the measured temperature. Hence, these ITM devices also need to be calibrated. For example, an electromagnetic pyrometer may be used to measure the intensity of the substrate's emitted radiation (e.g., photoluminescence) which may be correlated to substrate temperature. In general, a substrate may absorb electromagnetic radiation of some frequency and then emit radiation at another frequency corresponding to the substrates specific structure, composition and quality.
There are generally several methods of calibration. One in particular involves comparing a first substrate temperature in-situ as measured by the ITM device, to a second temperature of the same substrate as measured by a more accurate calibration device. However, as a portion of the calibration device generally needs to physically contact the substrate, which is discouraged, since the calibration device is generally not suited for normal substrate processing. For example, prior to running a batch of a particular substrate configuration in a particular process, the ITM device is calibrated with a calibration device. Once calibration occurs, normal substrate processing begins.
Phosphor thermometry is commonly used to calibrate ITM devices. By first transmitting light (electromagnetic radiation) to the inorganic phosphor within a first wavelength range, and then measuring the rate of decay of the fluorescent response within a second wavelength range, the temperature of the phosphor, and hence the substrate on which it is placed, can be determined. Again, as the phosphor must physically contact the substrate, phosphor thermometry is generally discouraged for use with production substrates.
In general, a phosphor is a fine powder that is doped with trace elements that when excited with short wavelength light (ultraviolet or blue), emits light at a longer wavelength. Ceramic phosphors are generally preferred in plasma processing chambers because of their tolerance of extremely high temperatures. A ceramic phosphor is generally inorganic, nonmetallic, and crystalline (e.g., Y3A15O12 (YAG) doped with Eu, Dy, or Tm, Y2O3 doped with Eu, or similar rare earth compounds).
In the case of a calibration substrate, the phosphor particles may be attached to the substrate surface, commonly the substrate side facing the chuck, and illuminated with a combination laser/sensor that is positioned in a cavity within the chuck. In general, the calibration substrate may have a special notch on the substrate surface where the phosphor particles are placed. In some configurations, the phosphor particles are directly attached to the substrate using a binder material. As with paint, a binder is a material which tends to provide a uniform consistency and solidification to the phosphor particles, and as well as cohesion with the substrate surface itself. In another configuration the phosphor particles are embedded in a patch that in turn is attached to the substrate. Normally, phosphor particles combined with a binder, or placed on a phosphor patch, are placed in a recessed position in the notch in order to prevent interference with the chucking process which secures the substrate on the chuck by electrostatic forces.
Referring now to FIG. 1, a common phosphor thermometry configuration as used in substrate manufacturing is shown. Generally, an appropriate set of gases is flowed and ionized to form a plasma 110, in order to process (e.g., etch or deposit) exposed areas of substrate 114, such as a semiconductor substrate or a glass pane, positioned on chuck 116. Substrate 114 is further configured with a phosphor material 140 as previously described. An optical fiber sensor/transmitter 142 may further be positioned such that a laser may be transmitted to phosphor material 140, and the resulting fluorescent response measured. Further coupled to sensor/transmitter 142 may be data acquisition and analysis device 144 that can record the observed the fluorescent response and correlate it to an approximate substrate temperature.
Referring now to FIG. 2, a simplified diagram of a capacitively coupled plasma processing system with a phosphor thermometry is shown. Generally, capacitively coupled plasma processing systems may be configured with a single or with multiple separate RF power sources. Source RF, generated by source RF generator 134, is commonly used to generate the plasma as well as control the plasma density via capacitively coupling. Bias RF, generated by bias RF generator 138, is commonly used to control the DC bias and the ion bombardment energy. Further coupled to source RF generator 134 and bias RF generator 138 is matching network 136, which attempts to match the impedance of the RF power sources to that of plasma 110. Other forms of capacitive reactors have the RF power sources and match networks connected to the top electrode 104. In addition there are multi-anode systems such as a triode that also follow similar RF and electrode arrangements.
Generally, an appropriate set of gases is flowed through an inlet in a top electrode 104 from gas distribution system 122 into plasma chamber 102 having plasma chamber walls 117. These plasma processing gases may be subsequently ionized to form a plasma 110, in order to process (e.g., etch or deposit) exposed areas of substrate 114, such as a semiconductor substrate or a glass pane, positioned with edge ring 115 on chuck 116, which also serves as an electrode. In addition, chuck 116 may also be configured with a cavity such that optical fiber sensor/transmitter 142 may transmit a laser to phosphor material 140, and measure the resulting fluorescent response. Furthermore, vacuum system 113, including a valve 112 and a set of pumps 111, is commonly used to evacuate the ambient atmosphere from plasma chamber 102 in order to achieve the required pressure to sustain plasma 110.
In addition, some type of cooling system (not shown) is coupled to chuck 116 in order to achieve thermal equilibrium once the plasma is ignited. The cooling system itself is usually comprised of a chiller that pumps a coolant through cavities in within the chuck, and helium gas pumped between the chuck and the substrate. In addition to removing the generated heat, the helium gas also allows the cooling system to rapidly control heat dissipation. That is, increasing helium pressure subsequently also increases the heat transfer rate.
In general, the prompt fluorescence decay time τ varies as a function of temperature and may be defined by:
                    I        =                              I            0                    ⁢                      exp            ⁡                          [                              -                                  t                  τ                                            ]                                                          EQUATION        ⁢                                  ⁢        1            where I is the fluorescence light intensity (−), I0 is the initial fluorescence light intensity (−), t is the time since cessation of excitation (s), and τ is the prompt fluorescence decay time (s). The unites of fluorescence light intensity are arbitrary. The time needed to reduce the light intensity to e−1 (36.8%) of its original value may be defined as the prompt fluorescence decay time. (See, Advances In High Temperature Phosphor Thermometry For Aerospace Applications, by S. W. Allison et. al., American Institute of Aeronautics and Astronautics, p. 2).
Referring now to FIG. 3, a simplified diagram of the lifetime decay characteristics of established phosphors over a temperature range extending from 0° K to 1900° K for the main emission lines of each. (See, Fiber Optic Temperature Sensor for PEM Fuel Cells, by S. W. Allison, Oak Ridge National Laboratory, U.S. Department of Energy, p. 7).
Referring now to FIG. 4, a simplified diagram of a substrate in which phosphor particles are attached to a substrate surface with a binder, such a silicone adhesive. As previously described, phosphor particles may be directly attached to substrate 108 using a binder material 140a in notch 406 that is generally positioned on the substrate surface away from plasma 112. In this example, the width 402 (along the lateral axis) is 0.25 inches, while the height 404 (along the perpendicular axis) is about 0.006 inches.
Referring now to FIG. 5, a simplified diagram of a substrate with a phosphor patch. As previously described, the phosphor patch 140b may be directly attached to substrate 108 using a thermally conductive silicone adhesive 505 in notch 406 that is generally positioned on the substrate surface away from plasma 112.
Although generally shielded between substrate and the chuck, ceramic particles in a substrate notch may still be exposed and hence etched by a plasma. Free to combine with other organic and inorganic byproducts generated by the plasma process, non-volatile ceramic particles may be deposited on the interior surfaces of the plasma chamber. These byproducts may eventually flake and increase susceptibility of substrate defects, reduce the mean time between cleaning (MTBC), reduce yield, etc.
In view of the foregoing, there are desired to have an apparatus for determining a temperature of a substrate while minimizing plasma chamber contamination and methods therefor.